Pneumocystis carinii (PC) is a significant cause of pneumonia in AIDS patients and other immunocompromised hosts. Particularly in the latter case, as well as in AIDS patients being treated for PCP, the number of organisms present may be very few. Finding these rare cysts is often time-consuming and difficult. It would thus be of benefit to both patient-care and laboratory efficiency to find a simple method of concentrating Pneumocystis, particularly in bronchoalveolar lavage specimens. If this works well, it may also be useful in concentrating the organism in sputum. The Clinical Center, as well as other hospitals, would benefit by the development of such a technique.